This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This protocol uses lactation as a physiological model of bone loss during estrogen deficiency and subsequent recovery of bone density after weaning. By studying lactation, there is the opportunity to identify optimal conditions for bone mass remodeling and formation. The overall hypotheses of this proposal are that 1) the skeletal response to lactation is mediated through the elaboration of one or more anabolic hormones (IGF-1) in conjunction with the return of adequate estradiol production;and 2) these events lead to both quantitative (BMD) and qualitative (bone geometry) changes in the skeleton that may confer a structural advantage for women later in life. This prospective cohort study will be conducted in postpartum women to assess hormonal determinants of bone density losses and recovery and an assessment of bone structural parameters using DXA and pQCT and/or MRI. recovery and an assessment of bone structural parameters using DXA and pQCT.